Nelson, 70, will return to Washington as the only Democrat in statewide office in Florida and with a goal, he said, of breaking the partisan gridlock that has marred progress for the past two years.

"In this toxic atmosphere, you must understand that Connie Mack is my opponent. He is not my enemy,'' Nelson told an enthusiastic crowd about 9 p.m. Tuesday. He told supporters he will "try to reach across the aisle and build consensus so that we can govern this country."

His victory comes despite being outspent by his challenger in a $39 million race, the most expensive of his political career.

Mack, 45, had hoped to unseat Nelson for the job once held by his father, Connie Mack III, who retired in 2000 and was replaced by Nelson. But the Fort Myers congressman never could get ahead of Nelson in the polls, despite an estimated $22 million spent by Mack and outside political groups.

Mack addressed his supporters at 9:30 p.m. and told them that it was "with some sadness tonight that we didn't win but, I'll tell you this: I'm very proud of the campaign we ran.''

"This is not an ending," he said. "This is just a beginning."

Mack's entry into the race was highly anticipated among Republicans as part of a multi-state strategy to win the four seats needed to retake the U.S. Senate.

He cleared the Republican field of two challengers and handily won the primary against former U.S. Rep. Dave Weldon, R-Indialantic. But his general election campaign struggled to raise cash as Nelson blistered Mack early with a barrage of negative ads.

Nelson's ads attacked Mack for his hard-partying youth and financial troubles even before the primary ended. Mack fought back by linking Nelson to President Barack Obama and a series of unpopular votes on health care and the federal debt.

Outside conservative groups, affiliated with Karl Rove, the Koch brothers and the U.S. Chamber of Commerce, funneled an estimated $15.2 million into the race, according to the nonpartisan Campaign Finance Institute.

Nelson, by contrast, spent a total of $17.5 million, including more than $12 million he raised himself and another $4.8 million from the Democratic party and third-party groups.

According to early polls, Nelson secured his win despite the surge by Republican Mitt Romney in Florida by luring crossover voters with his reputation as a moderate.

He said he would use that approach to end the bitter, ideological divisions that have stalled progress on the budget and deficit reduction in Washington.

His longtime friend Bruce Smathers predicted that Nelson will return to Washington in a stronger position than ever to help bridge the partisan differences. Nelson has begun working with moderate Republicans, he said, on a compromise over the budget deficit talks to avoid the so-called "fiscal cliff."

Nelson campaigned with Obama on Sunday in Hollywood, the first time the two have made an appearance together despite a number of trips by the president into the state. Mack, by contrast, campaigned aggressively with Romney and his surrogates throughout the state, hoping that Romney's success in Florida would translate into a victory for him.

It was not the only race Mack was watching closely on Tuesday. His wife, California U.S. Rep. Mary Bono Mack, was also awaiting results of her own close election fight.

Mack's father, the former senator, said his son was prepared for the outcome.

WASHINGTON --- Sen. Mitch McConnell on Tuesday officially pulled the plug on the latest plan to repeal the health care law, telling senators they will not vote on the measure and effectively admitting defeat in the last-gasp drive to fulfill a core promise of President Donald Trump and Republican lawmakers.